A. Technical Field
The present invention relates to a continuous production process for a water-absorbent resin powder and a powder surface detector used therefor. More specifically, the invention relates to a continuous production process for a surface-modified water-absorbent resin powder and a powder surface detector used therefor wherein the production process is a process for continuously producing a surface-modified water-absorbent resin powder comprising a polymerizing step, a drying step, a pulverizing step, a classifying step, and a surface-modifying step, and further, conveying steps of connecting these steps, and enhances the surface-modifying effect on the water-absorbent resin to give the surface-modified water-absorbent resin powder exhibiting a high absorption capacity, both without load and under a load, and a high saline flow conductivity.
B. Background Art
In recent years, water-absorbent resins are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids. Examples of the water-absorbent resins include: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, or crosslinked products of these hydrolyzed products; and crosslinked polymers of cationic monomers.
As properties which the above water-absorbent resins should have, there are generally cited such as excellent water absorption quantity, water absorption rate, gel strength, gel liquid permeability, and suction force to suck up water from base materials containing aqueous liquids such as body fluids, when the water-absorbent resins contacts the aqueous liquids.
Various water-absorbent resins, combing these properties and displaying excellent performance when they are used for sanitary materials (e.g. disposable diapers and sanitary napkins), have been proposed hitherto.
As to a method for improving absorption properties (e.g. absorption capacity without load and absorption capacity under a load) of a water-absorbent resin with good balance, there are known arts in which surfaces of water-absorbent resin powder are modified, for example, arts for modification in which: surfaces of water-absorbent resin particles are crosslinked, or a water-insoluble powder is attached to surfaces of water-absorbent resin particles, or deodorants, antibacterial agents, durability-enhancing agents, or other additives are attached to water-absorbent resin powder.
As to the surface-crosslinking art, it is known from such as JP-A-180233/1983, JP-A-016903/1986, JP-A-189103/1984, JP-A-117393/1977, JP-A-136588/1976, JP-A-257235/1986, JP-A-007745/1987, JP-A-211305/1986, JP-A-252212/1986, JP-A-264006/1986, DE 4020780, and JP-A-315216/1999. As to the water-insoluble-powder-attaching art, for example, JP-A-012367/1999 discloses a process in which fine silica powder or fine organic powder is added to water-absorbent resin powder. As to the art for modification in which deodorants, antibacterial agents, durability-enhancing agents, or other additives are added to water-absorbent resin powder, it is for example disclosed in JP-A-005847/1999, JP-A-267500/1999, and JP-A-315148/1999.
By the way, in recent years, as the production output of water-absorbent resins increases, each step of their production is more and more made continuous. However, the tendency is toward that, as water-absorbent resins having higher properties are demanded, such as surface-modifying step or fine-powder-recovering step is more and more added, therefore the number of steps increases and the production line becomes longer. Also in terms of properties, there is demanded a water-absorbent resin powder having a very narrow particle diameter distribution or a water-absorbent resin powder having a high water absorption capacity and a low water-extractable content, and further in recent years, it is being demanded that such as absorption capacity under a load or liquid permeability under a load should need to be high.
However, when attempts are made to obtain water-absorbent resins having such high properties with high productivity, it is very difficult to stabilize the production with high properties, and generally, the high properties and the high productivity conflict with each other in continuous production. Generally, the properties have hitherto tended to be deteriorated if the production output (scale) is raised. That is to say, in a process for continuously producing a surface-modified water-absorbent resin powder comprising a polymerizing step, a drying step, a pulverizing step, a classifying step, and a surface-modifying step, and further, conveying steps of connecting these steps, it has hitherto been very difficult to continuously produce a water-absorbent resin having a narrow particle diameter distribution and high properties with high productivity.
A. Object of the Invention
Considering the present circumstances as mentioned above, an object of the present invention is to provide:
a process for continuously producing a water-absorbent resin having a narrow particle diameter distribution and high properties with high productivity in a process for continuously producing a surface-modified water-absorbent resin powder comprising a polymerizing step, a drying step, a pulverizing step, a classifying step, and a surface-modifying step, and further, conveying steps of connecting these steps; and
a powder surface detector used for the above objective process.
B. Disclosure of the Invention
As a result of various study to solve the above problems and of repeated trials and errors, the present inventors have found out that: for the purpose of continuously producing a water-absorbent resin powder of high quality with high productivity by a process comprising a polymerizing step, a drying step, a pulverizing step, a classifying step, and a surface-modifying step, and further, conveying steps of connecting these steps, it is the most important to control a flow rate of the water-absorbent resin powder, and it is enough to supply a constant quantity of water-absorbent resin powder after adroitly making such control.
In addition, the present inventors have made arrangements such that: the above conveying steps include at least two hoppers, and the above quantifying-and-supplying hopper include: a supplying part which is placed in an upper portion of a tank for storing a water-absorbent resin powder and serves for supplying the powder into the tank; a discharging part which is placed in a lower portion of the tank and serves for quantifying the water-absorbent resin powder in the tank and discharging it therefrom; and a detector for detecting the quantity of the powder in the tank. In this way, the present inventors have succeeded in connecting the above findings to practical technique, and have completed the process according to the present invention and further a powder surface detector which is convenient to carry out this process.
That is to say, a continuous production process for a water-absorbent resin powder, according to the present invention, is a process for continuously producing a surface-modified water-absorbent resin powder comprising a polymerizing step, a drying step, a pulverizing step, a classifying step, and a surface-modifying step, and further, conveying steps of connecting these steps, and is characterized in that:
the water-absorbent resin powder is obtained by a process including the step of polymerizing an unsaturated monomer and has a crosslinked structure, and has a mass-average particle diameter of 300 to 600 xcexcm, and includes particles having particle diameters of 850 to 150 xcexcm in a ratio of not less than 90 mass % in particle diameter distribution, and has a property of displaying an absorption capacity of not less than 25 g/g without load, and has a water-extractable content of not more than 25 mass %; and
the conveying steps include at least two hoppers for storing and discharging the water-absorbent resin powder after the pulverizing step wherein at least one of the hoppers includes:
a tank for storing the water-absorbent resin powder;
a supplying part which is placed in an upper portion of the tank and serves for supplying the water-absorbent resin powder into the tank;
a discharging part which is placed in a lower portion of the tank and serves for discharging the water-absorbent resin powder from the tank; and
a detector for detecting the quantity of the powder in the tank.
The aforementioned at least two hoppers are usually a buffering hopper for storing and discharging the water-absorbent resin powder and a constant-quantity-supplying hopper which is placed downstream of the above buffering hopper and serves for quantifying and discharging the water-absorbent resin powder.
The above process according to the present invention can further comprise a second conveying step for conveying an additive powder for modifying the aforementioned water-absorbent resin powder, wherein the aforementioned second conveying step includes a constant-quantity-supplying hopper for quantifying and discharging at least the aforementioned additive powder as a hopper for storing the additive powder, wherein the aforementioned constant-quantity-supplying hopper includes:
a tank for storing the additive powder;
a supplying part which is placed in an upper portion of the tank and serves for supplying the additive powder into the tank;
a discharging part which is placed in a lower portion of the tank and serves for discharging the additive powder from the tank; and
a detector for detecting the quantity of the powder in the tank.
In addition, a powder surface detector, according to the present invention, is a detector provided to a tank for storing an additive powder as a superfine powder in order to detect a powder surface rising when accumulating the powder in the tank, and comprises:
a float which is supported movably up and down above the powder surface and of which the bottom contacts the powder surface when the powder surface has risen;
a hanging line for hanging down the float to support it movably up and down; and
a limit switch having a mover which serves for supporting the hanging line and is moved by a change of the weight, as applied through the hanging line, of the float.